OrderBy on Enum.GetValues()

Question

I'm populating a DropDownList in MVC 4 from an enum and I want to order the enum values from largest to smallest. However, there doesn't seem to be a direct way of approaching this. Currently, I'm using this code to add to a dictionary with the key being the ID and the value being the display text:

var priorities = Enum.GetValues(typeof(Models.Priority)).OfType<Models.Priority>().ToList();

for (int i = priorities.Count - 1; i >= 0; i--)
{
    Models.Priority priority = priorities[i];
    prioritiesDictionary.Add((int)priority, "Priority " + ((int)priority).ToString());
}

I don't believe that putting enum values into a list and looping backwards is the most efficient method. There are only four values in the enum, but is there a better way to perform an OrderBy operation on what is returned from Enum.GetValues? I know it might be minimal performance impact doing it the way I am, but I want to know for larger enums.

Answer 1

Sounds like you just want:

var priorities = ((Models.Priority[]) Enum.GetValues(typeof(Models.Priority)))
                                          .OrderByDescending(x => x);

Or to avoid quite as many brackets:

var priorities = (Models.Priority[]) Enum.GetValues(typeof(Models.Priority));
var ordered = priorities.OrderByDescending(x => x);

It's not clear how your current code is helping you, by giving you a dictionary - but the above will definitely give you a sequence of enum values, ordered from highest to lowest. You don't need to cast to int, because enum values of the same type are already comparable to each other.

If you need a list, just call ToList() after the OrderByDescending call.

Answer 2

What about this:

Enum.GetValues(typeof(Models.Priority))
    .Cast<Models.Priority>()
    .OrderByDescending(x => (int) x)
    .ToList();

You shouldn't use OfType<T>() in this style of logic because that silently throws away non-matching items. In this case a non-matching item would indicate a serious error or misuse.

If the expectation is that all items will already be a certain type, then we use Cast<T>().

OfType<T> is a filter operation, it is intended for use in scenarios where we know some items can not be cast to the expected type. OfType<T> is therefore useful in lists that might contain null valued items.

OfType<T> is commonly used in expressions that operate over untyped or object enumerations like Type Attribute or when the enumeration is a base type and you only want objects of a specific implementation, or that implement a specific interface.